Microwave assisted chemistry is the term used to describe systems, apparatus, and methods in which electromagnetic radiation in the microwave frequency range is used to initiate, drive, or otherwise enhance chemical or physical reactions. Microwave assisted chemistry is particularly useful in heating materials that are responsive to microwave radiation because under most circumstances, the resulting heating takes place much more rapidly than it would if the reactions were initiated or accelerated using more conventional heating techniques such as convection or conduction heating.
Microwave assisted chemistry can be used in a variety of chemical processes including moisture determination, ashing, digestion, extraction, and others. Under some circumstances, these various techniques are preferably or necessarily carried out in sealed vessels which, because of the generation or expansion of gases inside, must be able to withstand high pressures.
As well understood by those familiar with the electromagnetic spectrum, the term "microwave" is often used generically to refer to radiation with wavelengths of between about 1000 and 500,000 microns (.mu.), and corresponding frequencies of between about 1.times.10.sup.9 and 5.times.10.sup.11 Hertz (Hz). These are arbitrary boundaries, however, and other sources refer to microwaves as having frequencies of between about 10.sup.8 Hz and 10.sup.12 Hz and wavelengths of between about 300 centimeters (cm) and 0.3 millimeters (mm). For commercial and consumer purposes in the United States, the available microwave frequencies are regulated by the Federal Communications Commission and are generally limited to certain frequencies such as 2450 megahertz (MHz). Because of the relatively long wavelength of microwave radiation, microwave assisted chemistry techniques are often carried out in closed vessels which are in turn placed inside a device that bears a superficial relation to a consumer microwave oven, but that is much more sophisticated in its source, waveguide, cavity, and control elements.
This application is also related to co-pending application Ser. No. 09/260,209 filed Mar. 1, 1999 for "Composite Sleeve For Pressure Vessels With Continuously Wound Fabric Reinforcement," the contents of which are incorporated entirely herein by reference. Other patents and pending applications that are illustrative of the types of reaction vessels to which the present invention can apply include U.S. Pat. Nos. 5,427,741 and 5,520,886, both of which are commonly assigned with the present invention. Another version is set forth in co-pending and commonly assigned application Ser. No. 09/062,858, filed Apr. 20, 1998, the contents of which are incorporated entirely herein by reference ("the '858 application").
The composite sleeve set forth in the '209 application has provided, along with its predecessors, the opportunity to greatly increase the reaction pressures at which microwave assisted chemistry can be carried out, while avoiding some of the disadvantages of earlier generations of reaction vessels. In particular, the enhanced performance and controlled, non-shattering failure characteristics of the composite vessels set forth in the '209 application and those related to it, have permitted microwave assisted chemistry to be carried out at pressures as high as 800 pounds per square inch (psi) in the reaction vessel. As set forth in the '209 application and its predecessors, higher pressures can be accommodated to a certain extent by surrounding the reaction vessel with both the composite sleeve and a frame which holds the vessel in place and which urges the vessel lid or cap tightly against the reaction vessel.
As work has progressed at these higher pressures, however, a newer problem has tended to occur. Specifically, because typical reaction vessels are formed of polymers (i.e., transparent to microwaves and resistant to chemical attack) they tend to distort under the extremely high pressures now being used. Furthermore, because the frame keeps the dimensions of the vessel somewhat restricted along the axial direction of the vessel, the distortion that occurs at high pressures tends to be seen as a radial distortion of the typically cylindrical reaction vessels. This radial distortion in turn tends to unseat the vessel lid or cap from the vessel leading to loss of the desired pressures, or of the reagents inside the vessel, or both. In some systems (e.g., the '858 application), the distortion is welcomed as a technique for self-release of high pressures. In other circumstances, however, the high pressure is desired and the vessel should remain closed. Stated differently, the success in developing vessels and systems that can operate at high pressures has raised new issues that must be addressed as the vessels distort under the high pressures.
Accordingly, a need exists for microwave transparent, chemically resistant reaction vessels, typically formed of polymers, that can take advantage of the composite sleeve and frame structure described in the '209 application and its predecessors, and yet which can also withstand the high radial pressures exerted from the interior of the vessel as the reactions proceed, and as the frame maintains the longitudinal dimensions of the vessel and cap relatively the same as they are before reaction occurs. Those familiar with microwave assisted chemistry, and in particular with the types of devices described in the '209 application and its predecessors, will recognize, of course, that the vessel and frame together distort somewhat in a longitudinal direction, but no more than is desired under the design parameters of the vessel and frame. As set forth in the '209 and '858 applications, the slight flexing of the frame, which in turn allows the cap to release slightly, can be desirable under some circumstances as a self-moderating method of controlling the pressure inside the reaction vessel. Such is fine when a certain self release is desired at a particular pressure, but is disadvantageous when the vessel must remain closed at higher pressures in order to encourage a reaction to proceed or to become completed.
Accordingly, a need exists for polymeric reaction vessels and caps that will remain sealed even as internal pressures inside the vessels urge them to distort.